…or to allow unsized T.

we now distinguish between pidfd_spawn support, pidfd-via-fork/exec and not-supported

binutils 2.40 is required by LLVM 19, as older versions do not
know about the zmmull extension.

I've had to backport some patches to glibc and gcc as well,
as they don't build with binutils 2.40. Alternatively, we could
also switch to glibc 2.35 and gcc 12 (I think). I figured we'd
want to avoid the glibc version change, but if that's fine for
riscv I can go with that instead.

When encountering an E0277, if the type and the trait both come from a crate with the same name but different crate number, we explain that there are multiple crate versions in the dependency tree.

If there's a type that fulfills the bound, and it has the same name as the passed in type and has the same crate name, we explain that the same type in two different versions of the same crate *are different*.

```
error[E0277]: the trait bound `Type: dependency::Trait` is not satisfied
 --> src/main.rs:4:18
  |
4 |     do_something(Type);
  |     ------------ ^^^^ the trait `dependency::Trait` is not implemented for `Type`
  |     |
  |     required by a bound introduced by this call
  |
help: you have multiple different versions of crate `dependency` in your dependency graph
 --> src/main.rs:1:5
  |
1 | use bar::do_something;
  |     ^^^ one version of crate `dependency` is used here, as a dependency of crate `bar`
2 | use dependency::Type;
  |     ^^^^^^^^^^ one version of crate `dependency` is used here, as a direct dependency of the current crate
note: two types coming from two different versions of the same crate are different types even if they look the same
 --> /home/gh-estebank/crate_versions/baz-2/src/lib.rs:1:1
  |
1 | pub struct Type;
  | ^^^^^^^^^^^^^^^ this type doesn't implement the required trait
  |
 ::: /home/gh-estebank/crate_versions/baz/src/lib.rs:1:1
  |
1 | pub struct Type;
  | ^^^^^^^^^^^^^^^ this type implements the required trait
2 | pub trait Trait {}
  | --------------- this is the required trait
note: required by a bound in `bar::do_something`
 --> /home/gh-estebank/crate_versions/baz/src/lib.rs:4:24
  |
4 | pub fn do_something<X: Trait>(_: X) {}
  |                        ^^^^^ required by this bound in `do_something`
```

Address rust-lang#22750.

A `rustc_const_stable` attribute by itself has nonintuitive purpose when
placed in a public module.

Separately, it would probably be okay to rename `const_strlen` to just
`strlen` to make it more clear this is our general-purpose
implementation of strlen now, not something specifically for const
(avoiding confusion like in PR 127444).

…r=fee1-dead

On trait bound mismatch, detect multiple crate versions in dep tree

When encountering an E0277, if the type and the trait both come from a crate with the same name but different crate number, we explain that there are multiple crate versions in the dependency tree.

If there's a type that fulfills the bound, and it has the same name as the passed in type and has the same crate name, we explain that the same type in two different versions of the same crate *are different*.

```
error[E0277]: the trait bound `Type: dependency::Trait` is not satisfied
 --> src/main.rs:4:18
  |
4 |     do_something(Type);
  |     ------------ ^^^^ the trait `dependency::Trait` is not implemented for `Type`
  |     |
  |     required by a bound introduced by this call
  |
help: you have multiple different versions of crate `dependency` in your dependency graph
 --> src/main.rs:1:5
  |
1 | use bar::do_something;
  |     ^^^ one version of crate `dependency` is used here, as a dependency of crate `bar`
2 | use dependency::Type;
  |     ^^^^^^^^^^ one version of crate `dependency` is used here, as a direct dependency of the current crate
note: two types coming from two different versions of the same crate are different types even if they look the same
 --> /home/gh-estebank/crate_versions/baz-2/src/lib.rs:1:1
  |
1 | pub struct Type;
  | ^^^^^^^^^^^^^^^ this type doesn't implement the required trait
  |
 ::: /home/gh-estebank/crate_versions/baz/src/lib.rs:1:1
  |
1 | pub struct Type;
  | ^^^^^^^^^^^^^^^ this type implements the required trait
2 | pub trait Trait {}
  | --------------- this is the required trait
note: required by a bound in `bar::do_something`
 --> /home/gh-estebank/crate_versions/baz/src/lib.rs:4:24
  |
4 | pub fn do_something<X: Trait>(_: X) {}
  |                        ^^^^^ required by this bound in `do_something`
```

Address rust-lang#22750.

Generalize `fn allocator` for Rc/Arc.

Split out from rust-lang#119761

- For `Rc`/`Arc`, the existing associated `fn`s are changed to allow unsized pointees.
 - For `Weak`s, new methods are added.

```@rustbot``` label +A-allocators

Add AMX target-features and `x86_amx_intrinsics` feature flag

This is an effort towards rust-lang#126622. This adds support for all 5 target-features for `AMX`, and introduces the feature flag `x86_amx_intrinsics`, which would gate these target-features and the yet-to-be-implemented amx intrinsics in stdarch.

Use pidfd_spawn for faster process spawning when a PidFd is requested

glibc 2.39 added `pidfd_spawnp` and `pidfd_getpid` which makes it possible to get pidfds while staying on the CLONE_VFORK path.

verified that vfork gets used with strace:

```
$  strace -ff -e pidfd_open,clone3,openat,execve,waitid,close ./x test std --no-doc -- pidfd
[...]
[pid 2820532] clone3({flags=CLONE_VM|CLONE_PIDFD|CLONE_VFORK|CLONE_CLEAR_SIGHAND, pidfd=0x7b7f885fec6c, exit_signal=SIGCHLD, stack=0x7b7f88aff000, stack_size=0x9000}strace: Process 2820533 attached
 <unfinished ...>
[pid 2820533] execve("/home/the8472/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/home/the8472/.cargo/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/usr/local/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/usr/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */ <unfinished ...>
[pid 2820532] <... clone3 resumed> => {pidfd=[3]}, 88) = 2820533
[pid 2820533] <... execve resumed>)     = 0
[pid 2820532] openat(AT_FDCWD, "/proc/self/fdinfo/3", O_RDONLY|O_CLOEXEC) = 4
[pid 2820532] close(4)                  = 0
```

Tracking issue: rust-lang#82971

Stabilize const_cstr_from_ptr (CStr::from_ptr, CStr::count_bytes)

Completed the pair of FCPs rust-lang#113219 (comment) + rust-lang#114441 (comment).

`CStr::from_ptr` is covered by just the first FCP on its own. `CStr::count_bytes` requires the approval of both FCPs. The second paragraph of the first link and the last paragraph of the second link explain the relationship between the two FCPs. As both have been approved, we can proceed with stabilizing `const` on both of these already-stable functions.

Update dist-riscv64-linux to binutils 2.40

binutils 2.40 is required by LLVM 19, as older versions do not know about the zmmul extension.

I've had to backport some patches to glibc and gcc as well, as they don't build with binutils 2.40. Alternatively, we could also switch to glibc 2.35 and gcc 10 (I think). I figured we'd want to avoid the glibc version change, but if that's fine for riscv I can go with that instead.

r? ```@cuviper```

try-job: dist-riscv64-linux